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    <script>
        /* <,>,<=,>=,==,!=
        返回boolean
         */
         var a=10;
         var b=20;
         console.log(a==b)
         console.log(a!=b)
         console.log(a!="true") //NaN和任何数据做！=比较时，始终返回true
         //isNaN 判断一个值是不是NaN
         console.log(isNaN(NaN))
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